Many coatings are used on surfaces which are employed in an outdoor environment. Such coatings are exposed to sunlight, moisture and the like under a variety of temperature conditions and must, therefore, be able to withstand such changes in climate. Among the preferred weatherable coatings are those based on acrylic polymers. However, after long exposures of time, even these coatings will suffer deterioration at a pace which is dependent upon the geographical area involved.
As a result, several additives have been incorporated into coating formulations to slow down the degradative process by protecting against hydrolytic attack. However, incorporation of such additives increases the cost of producing weatherable coatings. Thus, it would be desirable to produce a highly weatherable coating which did not require the presence of such expensive additives.
In addition, many of the weatherable coatings heretofore employed utilized relatively large amounts of volatile organic solvents. With the rising price of energy, as well as the increased appreciation of the deleterious effects of organic solvents on the environment, it would be desirable to produce a weatherable coating which employs only a minimal amount of volatile organic solvent.
Among the compositions which have been developed in response to such need for highly weatherble coatings are those disclosed in British Pat. Nos. 1,556,456, and 1,556,464 which contain copolymers consisting of 50-75 weight percent t-butyl acrylate; 5-30 weight percent styrene, vinyltoluene or methyl methacrylate; and 4-20 weight precent of acrylonitrile. However, these copolymers, which are cured by crosslinking with an aliphatic dicarboxylic acid, possess high glass transition temperatures as they have a Durran softening point of 90.degree.-120.degree. C. Thus, as is apparent to one skilled in the art, these coatings must be applied as either (1) powders or (2) low-solids solutions as high-solids coatings containing such polymers would be too highly viscous to be utile. Therefore coatings employing these polymers are comparatively uneconomical as the use of powders would necessitate the formation of relatively thick coatings (of at least about 1.5 mil) whereas a low solids solution would require the use of relatively large amounts of solvent.
It has now been surprisingly found that coating compositions which contain low molecular weight copolymers comprised of polyfunctional acrylates and t-butyl acrylate possess unexpectedly superior humidity resistance vis-a-vis coating compositions which do not contain copolymers possessing both of these monomers. Moreover, because of the low molecular weight of the copolymers of this invention, these coating compositions may be formulated in a high solids solution form.